1. Field of the Invention
The present invention relates to an injection molding of a synthetic resin, and a molding method and molding apparatus therefor. Particularly, it relates to a large-sized molding which is excellent in dimensional accuracy and has smooth appearance without sink marks or warpages.
2. Description of Related Art
An injection molded article of a synthetic resin (hereinafter an injection molding of a synthetic resin) is generally obtained by injection molding a heat-plasticized synthetic resin in a mold cavity. In order to obtain the desired injection molding, it is necessary to inject, with a high pressure, the synthetic resin into a mold which is clamped with a sufficient mold clamping force. In general, the mold clamping force requires 300 to 500 Kg/cm.sup.2 per projected area of a molding so that the force of an injection molding apparatus becomes extremely big. A mold clamping force of several thousand tons is generally required to mold a large-sized molding. If the large-sized molding can be produced with a small mold clamping force, this will be very economic.
As the injection molding becomes thin-walled and large, the injection pressure from the injection cylinder is not sufficiently transmitted to the end of a mold cavity. This is because the injection pressure applied to the synthetic resin in the injection cylinder is transmitted to the synthetic resin at the end of the mold cavity through the synthetic resin previously injected into the mold cavity. Therefore, the pressure is remarkably lost between a gate section and the end of the mold cavity. In the case of a large-sized molding apparatus, the injection pressure at the end of the mold cavity is often down to about one tenth of the pressure at the gate section. In the case of ordinary injection molding, the injection pressure of 1000 Kg/cm.sup.2 is often down to about; 100 Kg/cm.sup.2 at the end of the mold cavity. An average pressure between the gate section and the end of the mold cavity per projection area of the whole of the molding is from 300 to 500 Kg/cm.sup.2. Accordingly, the necessary mold clamping force is also from 300 to 500 Kg/cm.sup.2.
As seen from the above, the pressure loss is big and the pressure is not effectively transmitted in general injection molding methods. Accordingly, in order to produce a large-sized molding, it is necessary to use an injection molding apparatus having a high injection pressure and mold clamping force. Further, problems such as the warpage of the molding and inaccuracy in a molding dimension occur due to the big difference between the pressures at the gate section and at the end of the mold cavity.
It is generally difficult to apply a pressure, which is sufficient enough to prevent sink marks, on the whole of the molding. The molding is designed so as to prevent sink marks. This limits the variety of molding designs. For example, there has been popularly designed a molding having a thin-walled rib and a boss with a small diameter or a molding having ribs thinner than a molding surface. It is difficult to lighten the molding as a whole by making strong thick-walled ribs at only the part of the molding requiring strength while thinning the molding surface. It is also difficult to reduce the number of parts of the built-up molding by using big bosses.
As methods to solve the above problems, there have been proposed a foaming injection molding method wherein a foaming agent is incorporated with a synthetic resin for injection molding, and a gas assisted injection molding method wherein a gas is injected after the injection of a synthetic resin.
U.S. Pat. Nos. 4,824,732 and 4,923,666, etc. describe the gas assisted injection molding method in detail.
U.S. Pat. No. 4,923,666 proposes to form a gas channel with gas in a thick-walled section, e.g., at a rib root and the like to prevent sink marks which easily occur at thick-walled sections and to transmit the injection pressure to the end of the molding.
U.S. Pat. No. 4,824,732 discloses a method for producing a hollow molding with a non-plastic fluid which is discharged to the outside of the mold after molding. The fluid used is a gas or a gas-like fluid having a very low viscosity.
A molding method called a sandwich injection molding method has been widely known to the public. The method comprises injecting a first synthetic resin and a second synthetic resin into a mold cavity to fill it up and form a so-called sandwich structure having a skin-core structure. In this method, it is difficult that the injected second resin forms an injection pressure transmission channel such as a gas channel. Indeed, forming such a channel by the second resin has not been reported in the prior art.
U.S. Pat. No. 5,149,482 discloses a method for reducing the mold clamping force in the sandwich injection molding method by forming a mold cavity wherein a thick-walled fluid passageway is formed in a portion of a thin-walled section of the mold cavity. In this method, the difference in wall thicknesses between the thin-walled section and the thick-walled section should be made extremely large in order to let the second resin selectively go through the thick-walled fluid passageway. As a result, the amount of the second synthetic resin increases. This patent does not mention the difference in viscosities between the first synthetic resin and the second synthetic resin corresponding to a low molecular weight polymer in the present invention. However, as will be discussed below, the difference in viscosities is an important feature of the present invention. The invention of the U.S. patent has difficulties if the amount of the second synthetic resin is reduced to the amount of the low molecular weight polymer used in the present invention for reducing the mold clamping force sufficiently.
U.S. Pat. No. 4,140,672 discloses a method comprising injecting a synthetic resin into a mold cavity, subsequently injecting a low viscosity fluid to fill the mold cavity and then discharging the low viscosity fluid to the outside of the mold, in which method a sandwich structure of a skin-core structure is formed during molding process as well as in a sandwich injection molding method. However, there is no mention about an injection pressure transmission channel formed by a low viscosity fluid.
Heretofore, it has been believed that a fluid forming the injection pressure transmission channel is a gas or a gas-like fluid. The gas assisted injection molding method is an excellent molding method. In the gas assisted injection molding method, however, a high pressure gas like nitrogen gas having a pressure of 200 to 300 Kg/cm.sup.2 is employed so that various problems such as high expenditure on facilities and special safety precautions occur. Additionally, in Japan, various other steps are required in order to comply with Japan's High Pressure Gas Regulation Act. When gas pressure is increased to improve reproducibility of molding appearance, expenditure on facilities is further increased. The present invention has been carried out to solve such problems. The present invention provides a large-sized molding which is injection molded with low clamping force, has excellent dimensional accuracy and has smooth appearance without sink marks or warpages.